This invention relates to thermistors. It more particularly relates to a thermistor composition that provides a more stable temperature coefficient over a wide temperature range.
A thermistor composition consisting essentially of cobalt oxide, manganese oxide, and zinc oxide is disclosed in U.S. Pat. No. 3,652,463 Riddel, assigned to the assignee of the present invention. The Riddel patent discloses particular ternary ratios of such oxides that provide a high temperature coefficient (i.e. beta) for discrete thermistors. These particular compositions can be fired in air and have moderate electrical resistivity, high stability, and good mechanical properties. On the other hand, these compositions generally exhibit a temperature coefficient which changes about 5%-20% over a wide temperature range such as about 50.degree.-350.degree. F. In other words beta is not stable. This produces a corresponding error in temperature measurement based on thermistor resistance change, unless this deviation in beta is compensated. Such compensation would have to be made for each significant span in the temperature range measured. If a computer is used for temperature measurement, considerable memory and extensive computer time is required to perform this compensation. If beta is constant over the temperature range measured, temperature measurement can be done by computer much more simply and quickly.
I have found how to make the temperature coefficient of certain high beta compositions more constant over a wide temperature range without detrimental effects on resistivity, stability or mechanical properties. Temperature measurement with such compositions can now be more simply and quickly done.